Environmental Risk Assessments of Coal Ash Impoundments

Transcription

1 2017 World of Coal Ash (WOCA) Conference in Lexington, KY - May 9-11, Environmental Risk Assessments of Coal Ash Impoundments George M. Huddleston III, Kathy W. Webb, Heather H. Smith, and Michael M. Spacil SynTerra Corporation, 148 River Street, Ste. 220, Greenville, SC CONFERENCE: 2017 World of Coal Ash ( KEYWORDS: risk assessment, coal ash impoundment, remediation ABSTRACT Environmental risk assessment has application in remediation and management of coal ash impoundments. Traditionally, human health and ecological risk assessments in the United States have been conducted in response to managing hazardous waste sites under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), commonly known as Superfund. However, risk assessment as an investigative tool also has application in regulatory negotiation, protecting public health, and environmental litigation. Risk assessments determine whether and to what extent a site threatens human health and the environment. For humans, this absence of threat is a level at which ill health effects are unlikely and the probability of cancer is very small. For ecological receptors (wildlife), determining risk levels is more complex and is a function of the receptors of concern (e.g., protected species), the nature of adverse effects caused by contaminants, and the desired condition of the ecological resources. The output of risk assessments produces estimates of current and possible future risks if no actions are taken. Generally, risk managers use risk assessment information along with other site factors to select the best remediation strategies to manage risks to acceptable levels. Coal ash can contain constituents that, if present in sufficient concentrations and certain forms, can pose risks to human health and the environment if exposure occurs. As part of corrective action plans developed for coal fired power plants, human health and ecological risk assessments were conducted. This paper presents details of the risk assessment process as applied to site investigations and remediation plans for coal ash impoundments. INTRODUCTION Coal combustion residuals (CCR), commonly known as coal ash, are created when coal is burned by power plants, and is a major industrial waste generated in the United States and other countries. The Disposal of Coal Combustion Residuals from Electric Utilities final rule was signed in December 2014 and published in the Federal Register (FR) on April 17, The U.S. Environmental Protection Agency (USEPA) regulations for safe disposal of CCRs are based on a culmination of extensive study on the effects of coal ash on the environment and public health. The CCR rule establishes

2 technical requirements for CCR landfills and surface impoundments under the Resource Conservation Recovery Act (RCRA), the nation's primary law for regulating solid waste. The CCR regulations address risks from coal ash disposal, including release of contaminants into groundwater and to air as dust, as well as the catastrophic failure of coal ash surface impoundments. Additionally, the rule sets out recordkeeping and reporting requirements, including making information available to the public, and supports the responsible recycling of CCRs by distinguishing safe, beneficial use from disposal. Forty years prior to establishing the CCR rule, EPA published its first risk assessment document, the Quantitative Risk Assessment for Community Exposure to Vinyl Chloride 2, thus setting in motion a framework for characterizing chemical hazards to public health and the environment and for setting priorities for contamination cleanup. The following year, the significant document, Interim Procedures and Guidelines for Health Risk and Economic Impact Assessments of Suspected Carcinogens 3, signaled EPA s intent that "rigorous assessments of health risk and economic impact will be undertaken as part of the regulatory process." A general framework outlined a process for analyzing cancer risks of pesticides. The water quality criteria released in the 1980s were the first EPA documents describing quantitative procedures used in risk assessment. In 1983, the National Academy of Science (NAS) published Risk Assessment in the Federal Government: Managing the Process 4, the principles of which EPA continues to integrate into risk assessment processes even today. In 1989, EPA released the Risk Assessment Guidance for Superfund, Volume I: Human Health Evaluation Manual 5, followed by the Framework for Ecological Risk Assessment in Traditionally, these publications were applied to managing hazardous waste sites under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), commonly known as Superfund. They have laid way for numerous support guidance documents over the years and are still in use today. Environmental risk assessment as an investigative tool has application in remediation and management of coal ash impoundments, regulatory negotiation, protecting public health, public communications, and even environmental litigation. Coal ash can contain constituents that, if present in sufficient concentrations and certain forms, can pose risks to human health and the environment if exposure occurs. As part of corrective action plans developed for several sites, human health and ecological risk assessments were conducted. The risk assessment process applied to CCR site investigations and remediation plans for coal ash impoundments are presented in this paper. METHODS Screening and baseline (quantitative) human health and ecological risk assessments were conducted for certain coal ash impoundments as part of comprehensive site

3 evaluations for developing corrective action plans. Risk assessments followed federal, regional, and state protocols, which are outlined below. A human health risk assessment is the process conducted to estimate the nature and probability of adverse health effects in humans who may be exposed to chemicals in contaminated environmental media, now or in the future. Human health risk assessments described in this paper generally included: Planning and Scoping. Hazard Identification - Examined whether coal ash constituents had the potential to cause harm to humans, and if so, under what conditions. Dose-Response Assessment Examined the numerical relationship between exposure and effects. Exposure Assessment Examined what was known about the frequency, timing, and levels of contact with coal ash constituents. Typical exposure scenarios used in this case included construction workers, maintenance workers, off-site residents, and recreational users. Risk Characterization Examined how well data supported conclusions about the nature and extent of the risk from exposure to coal ash constituents. An ecological risk assessment is the process conducted for evaluating the likelihood that the environment may be impacted as a result of exposure to one or more environmental stressors. The ecological risk assessments described in this paper generally included: Planning and Scoping. Problem Formulation Gathered Information to help determine species of plants and animals that were potentially at risk and needed to be protected. This process included selection of receptors of interest (ROIs) that were unique to each site. Analysis - Determined what plants and animals were exposed and to what degree they were exposed, and if that level of exposure was likely or not to cause adverse effects. Typical ecological receptors included fish and terrestrial and aquatic birds and mammals, which were further divided into diet type (e.g., herbivores, carnivores, etc.)

4 Risk Characterization Included risk estimation (exposure and effects relationships) and risk description (information important for interpreting the risk results) and identified levels for harmful effects on receptors of interest. For both human health and ecological risk assessments, conceptual exposure models were developed, including the type of media affected, exposure routes and pathways, and human and ecological receptors that were specific to a site. Next, the screeninglevel assessments were conducted by comparing analytical results from groundwater, surface water, soil, and sediment to applicable media-specific screening values to identify constituents of potential concern (COPCs). If COPCs exceeded screening values, quantitative risk assessments were performed. Site-specific human health risk-based concentrations (RBCs) were developed for COPCs, exposure concentrations were derived, and the two were compared to draw conclusions about the significance of potential human risks at a site. For ecological receptors, exposure concentrations were compared to conservative ecological effects levels to draw conclusions about the possibility of potential ecological risks at a site. RESULTS Each CCR site varied in its respective COPC lists following the media-specific screening process. The screening process resulted in COPCs that warranted further investigation by quantitative risk assessments. While conceptual exposure models were developed early in the risk process to address potential source-migration pathwayreceptor linkages for coal ash-derived COPCs, the COPCs identified from the screening process were not always coal ash-derived. Many background concentrations of naturally occurring inorganic elements were greater than their conservative risk-based screening levels. The following list includes the most common COPCs retained among the CCR sites: Human Health COPCs: Arsenic Barium Cobalt Iron Lead Manganese Selenium Thallium Vanadium Zinc Ecological COPCs: Aluminum Copper Iron Lead Manganese Molybdenum Selenium Vanadium Zinc At each facility, on and off-site exposure scenarios were evaluated, where coal ash COPCs potentially posed unacceptable risks to humans and ecological receptors. In general, the human health risk assessments resulted in limited to no risk. Human

5 health risks estimated within site property boundaries were primarily associated with an adolescent trespasser scenario and a commercial/industrial worker scenario exposed to surface water seeping from ash basins and soils contacted by seeps. Off-site human health risks were observed at only a few of the sites, with very limited risk associated with recreational and subsistence fisherman ingesting fish tissue. Fish tissue concentrations were estimated from surface water concentrations using bioconcentration models, which conservatively estimate tissue concentrations and likely overestimate real risks. Ecological risks were evaluated for terrestrial and aquatic animals using very conservative effects levels from published literature. Limited to no ecological risks were observed at the sites, with the limited risks primarily within areas directly surrounding the ash basins. Based on the risk assessments, numerous COPCs were eliminated as having the potential to pose risks at the sites. Only a few COPCs remained for some of the sites after completion of the risk assessments. The following is a comprehensive summary of COPCs planned for further monitoring post remedy. Human Health COPCs: Arsenic Lead Zinc Ecological COPCs: Aluminum Molybdenum Vanadium DISCUSSION Environmental risk assessments contributed to proposed corrective actions for groundwater restoration at the coal ash impoundment sites where they were conducted. Without the risk assessments, proposed remedies may have been based primarily on comparison of coal ash constituent concentrations to state groundwater and soil standards and groundwater modeling. Comparisons to regulatory standards alone often omit context and consideration of site-specific conditions, as well as introduce greater uncertainty associated with the potential environmental risks. After all, exceeding a media-specific regulatory standard does not indicate that adverse effects will occur or have occurred. Rather, a concentration less than the standard indicates that adverse effects are unlikely to occur. Therefore, relying on exceeded standards presents many elements of uncertainty. For example, does a slight exceedance mean only minimal effects, or does a gross exceedance indicate widespread effects? While uncertainties are inherent in environmental investigations, risk assessment helps decrease uncertainties and focus appropriate resources towards remedy. The risk assessment-informed proposed corrective action plans for the CCR sites included cap-in-place closure with monitored natural attenuation (MNA), removal of ash, and active groundwater remediation. Because risks to human health and wildlife were not identified or were at most limited to very specific scenarios, the risk assessments supported MNA to some extent at all of the sites, thus decreasing the need to excavate

6 impoundments entirely. Certainly, other factors such as groundwater modeling were considered in corrective action plans. Where source control (ash removal) was proposed, material would be safely recycled or reused in a lined structural fill or disposed in a lined landfill. Where the potential for groundwater to migrate off site was identified and the risk assessment indicated possible exposure to coal ash constituents, active groundwater remediation was proposed in the form of interceptor trenches (groundwater collection trenches) or extraction wells at specific locations. Environmental risk assessments were conducted to assist in development of corrective action plans for remediation and management of CCR sites. By implementing standard risk assessment procedures, performing modeling, and using site-specific factors (e.g., geology, hydrogeology), utility owners and regulatory agencies can select the best remediation strategies to manage risks associated with coal ash impoundments to acceptable levels. REFERENCES [1] Disposal of Coal Combustion Residuals from Electric Utilities, 40 C.F.R. 257 and 261, [2] Kuzmack, A.M. and McGaughy, R.E. Quantitative risk assessment for community exposure to vinyl chloride, EPA Office of Planning and Management and Office of Health and Ecological Health, December [3] Train, R. Interim procedures and guidelines for health and economic impact assessments of suspected carcinogens, EPA Office of the Administrator, [4] National Research Council (NRC). Risk assessment in the federal government: managing the process, National Academies Press, [5] U.S. Environmental Protection Agency (USEPA). Risk assessment guidance for Superfund, Volume I: Human health evaluation manual (Part A), Interim Final, EPA/540/1-89/002, Office of Emergency Remedial Response, Washington, DC, [6] U.S. Environmental Protection Agency (USEPA). Framework for ecological risk assessment, EPA/630/R-92/001, Washington, DC, 1992.